1. Field of the Invention
The present invention relates to a focus-point selection system that selects one focus point from among a plurality of focus points to use in an autofocus operation for quickly bringing a desired subject into focus.
2. Description of the Related Art
The phase-difference autofocus function and the contrast autofocus function are commonly used autofocus methods that automatically bring a subject into focus on an imaging surface.
The phase-difference autofocus function has been the generally adopted autofocus method for single-lens reflex (SLR) digital cameras because of its effectiveness at quickly completing a autofocus operation. U.S. Pat. No. 6,453,124 discloses that in the phase-difference autofocus function, a focus lens is adjusted so that an optical image at a predetermined focus point in a photographic area is brought into focus on the imaging surface.
A recent SLR digital camera has a live view function that displays a real-time image of a subject on a monitor while a release operation is not underway. In order to carry out the live view function, an optical image of a subject must be captured by an image sensor.
An image signal generated by the image sensor upon capturing an optical image can be used not only for displaying a real-time image on the monitor, but also for analyzing an optical image. From the analysis of an optical image specific kinds of targets, such as a human face for example, can be automatically detected. It is proposed that an exposure adjustment operation and an autofocus operation be carried out for the detected target.
The location of a detected target does not generally coincide with a certain focus point at a predetermined location in the photographing area. Accordingly, in the case where a target to be photographed is automatically detected, the contrast autofocus function, which can bring a subject located anywhere in the photographing area into focus, is carried out instead of the phase-difference autofocus function.
However, the contrast autofocus function is problematic because its autofocus operation is relatively time consuming due its process of determining whether a subject remains in focus as the focusing lens is adjusted.